Twitchy Old Mare

From Helsingfors to Honolulu last week seismologists watched their instruments with sharp concern. Mother Earth seemed to be twitching all over like an old fly-pestered mare, sorely frightening millions of her inhabitants.

Turkey was rocked by three earthquakes, worst since 1874. The towers of Istanbul tottered against the sky. Townsfolk rushed jabbering through the streets, slept in the open despite a pouring rain.

Two quakes struck Holland's province of Limburg, shook inhabitants out of bed.

In Bulgaria, three temblors terrorized the districts of Plovdiv, Burgas, Borisovo.

Shocks felt on the island of Hawaii turned fearful eyes on great Kilauea whose volcanic belchings have always been heralded by earthquakes.

Three times within a week University of California seismographs registered tremors within 500 mi. of San Francisco.

Calcutta instruments recorded severe shocks, apparently north of Mt. Everest.

Strong quakes rocked three provinces of Argentina. In Buenos Aires Astronomer Martin Gil predicted violent upheavals within a week, a bad year for quakes in general, basing his prophecies on increase in sunspot activity.*

With the exception of Holland, which is a zone of moderate intensity, all the centres of last week's disturbance lay in the areas where severe earthquakes must be expected on the basis of past performance. The major earthquake belt is an irregular band crossing Eurasia from Spain to China and bordering the entire rim of the Pacific.

Earthquakes occur when the earth seeks relief from the strain of forces acting upon it--volcanic forces, shifts of pressure due to erosion, possibly tidal forces and the centrifugal pull of terrestrial rotation. There are vertical thrusts, sending up new islands to astonish mariners, building new mountains, deepening the seas; horizontal movements producing faults or sudden slips of rock masses along previously existing faults.

Earthquakes must obviously be of frequent occurrence since the stresses causing them never cease. Scattered over the globe are some 200 seismological stations which, if the poorest were equipped as well as the best, would record about 8,000 quakes every year--nearly one an hour. Of these some 70 are major quakes. Catastrophic shocks involving heavy loss of life average a little over one a year. Despite the safety of the U. S. east of the Rockies, this country had 62 quakes of moderate intensity in 1933. Safest place in the U. S. is New York City whose ancient rocks did their slipping and sliding aeons ago.

Three kinds of earthquake waves speed through the earth from the epicentre. Fastest are P (Push) waves which travel about 5 mi. per sec. They take a nearly straight path through the earth to the recording station. Then come the S (Shear) waves which make about 3 mi. per sec., follow the same path as the P waves. Last come the L (Long) waves which ripple around Earth's surface at about 2 mi. per sec. The transverse shear waves are the crux of an unsettled controversy about the nature of Earth's core. Some observers affirm they have recorded shear waves passing through the core, believe therefore that the core is solid. Others are equally sure the core refuses to transmit shear waves, is hence molten--at a pressure of 15,000,000 Ib. per sq. in., a temperature of 50,000 degrees.

The distance of the shocks from the recording station can be readily deduced from the difference in time of arrival of the P and S waves, since their speeds are known. Direction is much harder to determine, even with a half-dozen instruments lined up facing different points of the compass. Frequently it is necessary for three widely separated stations to compare their distance figures before the quake can be located.

*Most astronomers recognize no connection between sunspots and earthquakes.

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